Title

Author

Date of Award

5-2012

Degree Type

Thesis

Degree Name

Master of Science

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Yanfei Gao, Wei He

Abstract

As a potential structural material, performance of bulk metallic glasses (BMGs) under cyclic loading is critical in applications. Among the mechanical properties of metallic glasses, the fatigue behavior received less attention and is not well understood. BMGs exhibit significant size effects on the mechanical behavior under monotonic loading, such as compression, tension, and bending. At the same time, different test-volume could lead to a distinct difference in the mechanical behavior of BMGs.

The fatigue behavior of the Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 [atomic percent (at. %)] BMG alloy (Vitreloy 105) with different sizes has been investigated. The results indicate a size effect on fatigue behavior. The fatigue endurance increases with the increasing specimen thickness. Shear-band spacing was measured, and the apparent fracture toughness was calculated, both of them scale with the sample thickness. We suggest that this trend can be attributed to the reduced plasticity of the large-size sample, which is highly localized in shear bands. Reduced shear bands provide some benefits to the fatigue-initiation process.

Four-point bending and three-point bending fatigue tests were conducted on iron (Fe)-based bulk-metallic glasses (BMGs): Fe41Co7Cr15Mo14C15B6Y2 (atomic percent, at%). The experiments were performed, using an servo-hydraulic machine at a freuency of 10 Hz, with an R ratio (R = σ[sigma]min./σ[sigma]max., where σ[sigma]min. and σ[sigma]max. are the applied minimum and maximum stresses, respectively) of 0.1. The test environment is air. Under the four-point-bending fatigue test, the fatigue-endurance limit of Fe41Co7Cr15Mo14C15B6Y2 is approximately 450 MPa. However, there is a significant difference between the fatigue-endurance limit of Fe41Co7Cr15Mo14C15B6Y2 under three-point-bending and four-point-bending tests. Fatigue behavior exhibits test-volume effects. The fatigue-endurance limit increases with decreasing the test volume. Following the completion of fatigue tests, the samples were examined by scanning-electron microscopy (SEM) to identify the fatigue and fracture mechanisms. The fatigue-fracture morphology indicated that Fe41Co7Cr15Mo14C15B6Y2 has a brittle fracture mode.